: : : 20-20, more often written 20/20, is the standard for clear vision. When an eye doctor measures eyesight, a reading of 20/20 represents clarity. Hindsight, looking back on what has already happened, allows us to see clearly the course of action we should have taken, but perhaps didn't. So ... if you nag me with what I should have done yesterday, I will gently remind you that your hindsight is 20/20, but where was all this good advice when I needed it?

: : In the UK, although '20/20' is used in the above colloquial sense, if you have your eyes tested perfect vision is given 6/6 or, even better, 6/5. I have a vague memory that this is to do with angles needed to view a letter, but don't quote me!

: Sounds like 20 feet = 6 meters, maybe?

20/20 means that you can read letters on a chart 20 feet away that people with good vision can read 20 feet away. 20/200 means you can read letters on a chart 20 feet away that people with good vision can read 200 feet away: you're extremely myopic and legally blind. The standard for a driver's license in my state in 20/40. In practice, optometrists don't have 200-foot examination rooms. That's why the letters on the chart are different sizes.